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Donation Amount is available in the Paypal Payment tool.
You can see this option available during the configuration of the Paypal payment wizard.
You can find out more about order types form on following link:
And here is a JotForm template http://www.jotform.com/form-templates/donation-form-2 that have a basic example of the Custom Amount/Donation using Paypal tool.
Hope this will help. Let us know if you need further assistance.